Application and evaluation of the SWAP model for simulating water and solute transport in a cracking clay soil

In Sicily, the increasing scarcity of quality water is leading to irriga- from salinization (Crescimanno et al., 2004). tion with saline water in soils having a considerable susceptibility to In Sicily, the increasing scarcity of good quality water cracking. Irrigation systems involving high application rates are used coupled with intensive use of soil under semiarid to arid in these irrigated areas, and bypass flow during irrigation is thus climatic conditions, is leading to irrigation with saline prevalent. Adoption of management practices accounting for cracking water on soils having a high shrink-swell potential and is therefore necessary to prevent salinization and land degradation. susceptibility to cracking (Crescimanno and Provenzano, In this paper, water flow and solute transport in a Sicilian cracking 1999). These soils are irrigated in the summer season, soil irrigated with saline water was simulated by using the soil-water- when the cracks open up, by sprinkler systems, which inatmosphere- plant environment (SWAP) model, and the simulated volve high application rates. Because of these high apresults compared with measured values of soil moisture and salinity. plication rates, bypass flow, that is, the rapid transport The soil hydraulic parameters were obtained by inverse method based of water and solutes via macropores or cracks to subsoil on multi-step outflow experiments, adopting two different sets of hy- or to groundwater (Bouma, 1991; Crescimanno, 2001), draulic parameters/functions, that is, (i) the van Genuchten-Mualem, (VGM model) and (ii) the Brutsaert retention equation coupled with is prevalent during irrigation. the hydraulic conductivity model proposed by Gardner (B-G model). Laboratory investigations performed on undisturbed The results obtained using field measurements from four soil profiles soil columns sampled from these areas showed that saliof a cracking clay soil showed that SWAP provided accurate predic- nization or leaching occurred during bypass flow dependtions of water content, , when the soil hydraulic properties were ing on the concentration of the applied solution compared expressed according to the B-G model. Using the B-G hydraulic pa- with the concentration of the pore solution (Crescimanno rameters/functions, the model was calibrated with reference to the and De Santis, 2004); the efficiency of salt-leaching was dispersivity (Ldis).Acalibration value of about 20 cm was found for the found to depend on crack volume (Crescimanno et al., four different profiles. In the conditions occurring in the Sicilian area 2002). The low values of the sodium adsorption ratio where we are focusing our attention, the predictive errors associated (SAR) of irrigation water, and the low values of the exwith the simulated ECsat, can be considered acceptable if the purpose changeable sodium percentage (ESP) measured in these of application is to predict the influence of salinity on crop yield.